Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is a static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to a high voltage electrical power system and a DC side connected to a temporary electric power storage means such as capacitors. The STATCOM can supply reactive power to or absorb reactive power from the transmission line.
As opposed to the STATCOM, which only compensate for reactive power, another concept is to connect a dc power source to a compact STATCOM, and thus being able to perform active power compensation. The construction may be used e.g. as a spinning reserve and for compensating for fluctuating energy levels in the power system.
Today the dc power source is a high voltage battery. Since the power apparatus is connected to the ac voltage of the high voltage electrical power system, a plurality of battery cells have to be connected in series to match the dc voltage of the power apparatus. Moreover, to obtain the desired active power and duration of the energy storage, a number of strings including a plurality of battery cells must be connected in parallel.
Furthermore, in case of a fault, such as a short-circuit, the power apparatus has to be protected. Consequently, two switches are provided in each string to disconnect the string in case the converter, a string, or a part of the string is short-circuited. One switch is able to disconnect the string from the positive dc rail of the converter and the other switch is able to disconnect the string from the negative dc rail of the converter. Each of the switches has to be rated for the entire converter voltage to be capable of protecting the power apparatus.
Up to now, mechanical dc circuit breakers have been used as switches for connection and disconnection of the strings. However, the availability of dc circuit breakers is limited and the ratings of these breakers are considerably low and less than the required voltage ratings in high voltage electrical power systems. To overcome this problem mechanical ac circuit breakers with some additional circuitry, such as a resonance circuit connected in parallel with the ac circuit breaker, have been used.
A solid-state switch is an electronic switch that, unlike a mechanical breaker, contains no moving parts. There is a desire on the market to be able to exchange the mechanical ac circuit breakers with solid state switches. However, a problem with most of the commercially available solid-state switches is that they become an open-circuit when failed, which means that if the solid-state switch fails, the string which includes the failed switch will be disconnected. As a consequence the dc power source will have its capability significantly reduced.